1. Field of the Invention
This invention relates to a copying apparatus of the type in which an original to be copied is scanned and a photosensitive medium is slit-exposed to the image of the original, and more particularly to a variable magnification copying apparatus whose copying magnification can be changed.
2. Description of the Prior Art
To change the copying magnification in a scanning type variable magnification copying apparatus, the image formation magnification of the optical image of an original projected upon a photosensitive medium must be changed and also the velocity ratio of the original scanning velocity to the velocity of the photosensitive medium must be changed correspondingly to a selected copying magnification. Described more particularly, assuming that the original scanning velocity is V.sub.1 and the velocity of the photosensitive medium is V.sub.2, the velocity ratio V.sub.2 /V.sub.1 is changed in accordance with the following equation: EQU V.sub.2 /V.sub.1 =m
where m is a selected magnification. By changing the aforementioned velocity ratio in accordance with the above equation, the magnification of the copy image with respect to the direction of movement of the photosensitive medium, namely, the copying magnification with respect to the original scanning direction, can be rendered into a selected magnification. The magnification of the copy image with respect to a direction perpendicular to said direction is changed by changing the projection magnification of the optical image, as mentioned above.
Generally, in most apparatus, the aforementioned velocity ratio is changed by changing the original scanning velocity. For example, U.S. Pat. No. 3,884,574 discloses an original scanning velocity changing device which comprises a combination of a plurality of gear sprockets, a chain and a plurality of electromagnetic clutches. In such a device, however, at least the same number of electromagnetic clutches as the number of copying magnifications which can be selected is required. This not only makes the construction of the device complicated, but also the length of time from the point of time at which the main switch is closed till the point of time at which operation is started is irregular between individual clutches and such irregularity results in the irregularity of the point of time at which image formation is started.
U.S. application Ser. No. 141,922 describes a copying apparatus in which the velocity of the photosensitive medium is changed to change the copying magnification. Again in this apparatus, the velocity of the photosensitive medium is changed correspondingly to a selected magnification by a plurality of gears and a plurality of electromagnetic clutches, and this leads to an inconvenience similar to that described just above.